The invention relates to a resistance welding method.
During resistance welding using direct current, the welding electrodes are heated dissimilarly, whereby the welding electrode with positive polarity gets warmer than the welding electrode with negative polarity. This is due to the Peltier heat, and Joule""s heat. The heating of the electrode with positive polarity on one side causes a reduction of serviceable life or endurance of this electrode. This is especially true when welding material with high heat conductivity, such as aluminum.
In order to avoid these disadvantages, it is known to use alternating current for welding.
A method and a device for resistance welding with alternating current is known from DE 41 13 117 C1. With this method a sequence of positive medium frequency current pulses is first created on the primary side of a welding transformer, and then a sequence of negative medium frequency current pulses is created and transferred to the welding electrodes connected to the secondary side of the transformer, so that the welding current is an alternating current. One disadvantage of the known method exists in the fact that the welding transformer not only has to transfer the medium frequency voltage pulses, but also the resulting current using low frequency, because the welding alternating current is gained on the primary side of the transformer. The welding transformer required for this is large and heavy, as well as costly to produce. Fundamentally, the welding alternating current must not have a rectangular shape in the known device, because the transfer behavior will not allow such a shape.
Similar devices are known from DE 30 05 083 C2 and EP 0 261 328 A1.
A method for resistance welding is known from DE 44 40 351 C1, with which a direct voltage applied to the welding electrodes is commutated in order to produce a welding alternating current, resulting in an alternating current with the positive and negative half waves essentially in a rectangular shape. According to the printed publication, this should avoid the welding spot being shifted toward the warmer positive electrode, and is therefore arranged in an asymmetrical way at a contact plane, in which the work pieces to be welded are making contact. This interferes with the stability of the welding joint. Further, the method known from the printed publication is to prevent uneven wear of the welding electrodes, which results in a reduction of serviceable life or endurance of the electrodes.
A device for the operation of the method known from DE 44 40 351 C1 is known from DE 295 08 868 U1.
A method for resistance welding of nails in an automatic nail machine is known from U.S. Pat. No. 4,409,461, with which a number of nails is fed to the welding electrodes of a welding unit by way of a rotating disk, and welded to a pair of wires. In this known method, a direct voltage applied to the welding electrodes is commutated using no control means before each welding operation.
The invention is based on the task of stating a process of resistance welding, with which high serviceability or endurance of the electrodes can be achieved, and which is simple to perform.
This task is solved as set forth below.
According to the invention, the polarity of the direct voltage at the welding electrodes is selected before or after each welding operation by a control means according to at least one parameter detected during the welding process. A welding process according to this invention is the operation, which produces a welding joint between the two work pieces to be welded, i.e., a welding point or a welding seam.
For example, if the measured temperature of both welding electrodes, hereinafter referred to as electrodes, shows that the positive electrode was heated more than the negative electrode, and that the temperature difference between the two electrodes exceeds a predetermined value, the direct voltage at the electrodes is then commutated before the next welding operation, for example before producing the next welding point, so that the electrode with an initially negative polarity, and now an electrode with positive polarity is now heated at a higher temperature.
It has unexpectedly been proven that the method according to this invention can increase the serviceable life or endurance substantially, without the necessity of commutation during the welding operation, as known from DE 44 40 351 C1.
According to the method relating to the invention, an even wear of the electrodes can be achieved. This will prevent premature wear and reduces setup time necessary due to exchanging the electrodes. In this way, the welding process can be operated at less cost. In addition, the method relating to the invention is simple, and can be produced with limited use of equipment.
Detecting at least one parameter of the welding process consists of the detection of a parameter or the parameters during the welding process, for instance during the creation of a welding point, as well as the detection of a parameter of the parameters before or after the actual welding operation. For example, it is possible to detect the temperature of the welding electrodes before, during, or after the welding operation. It is also possible, to detect at least one parameter during the welding operation, and at least one other parameter before or after the welding operation.
Further improvement of the teaching according to the invention includes the possibility that at least one parameter of the welding process is measured, and can then be used for selecting the polarity of the direct voltage at the electrodes. With this design, the selection of the polarity therefore occurs according to a measured parameter of the welding process. According to the invention, the parameter(s) can be measured during the welding operation, for example during the creation of a welding point. However, the parameters can also be measured before and/or after the welding operation.
According to further improvement of another design, another parameter is calculated from at least two measured parameters of the welding process, and used for selecting the polarity of the direct voltage at the welding electrodes. With this design, not only the measured parameters, but also the parameters derived from the measured parameters can be used for selecting the polarity.
According to the invention, any suitable parameters of the welding process can be used as the basis for the selection of the polarity before or after each welding operation. Preferably, the parameters of the welding process should include at least the following:
the temperature of the welding electrodes, and/or
the degree of wear at the welding electrodes, and/or
the total duration of the welding time, during which the welding operation took place at unchanged polarity since a previous commutation of the direct voltage, and/or
the number of welding joints, especially the welding points or welding seams created at unchanged polarity since a previous commutation of the direct voltage, and/or
the ratio of duration, during which the polarity of an electrode was positive for the creation of a number of welding joints at the duration, during which the polarity of the electrode was negative for the creation of a number of welding joints, and/or
the thickness and/or the material of the work pieces to be welded.
These parameters by themselves, or in combination with each other, represent a reliable basis for the selection of the polarity of the electrodes.
An additional beneficial improvement of the design includes the possibility to select the polarity of the direct voltage at the electrodes during the creation of a number of welding joints, or during a predetermined time interval in such a way, that the sum of the welding times, during which the polarity of an electrode is positive, is essentially the same sum as the welding times, during which the polarity of the electrode is negative. With this design, the electrodes are therefore xe2x80x9cstressedxe2x80x9d with the same amount of positive, and negative polarity, so that an uneven wear can be avoided reliably.
Another improvement of the method according to the invention includes the possibility to externally enter at least one parameter of the welding process into an input device connected to the control means. With this design, data such as thickness and/or the material of the work pieces to be welded can, for instance, be externally entered by an operator and used by itself, or in combination with additionally measured parameters as the basis for the selection of the polarity of the direct voltage of the welding electrodes.
The degree of wear of the welding electrodes can be detected in any suitable way. According to the improvement of the design, the degree of wear of the welding electrodes is detected with the use of optical means.
The temperature of the welding electrodes can be detected in any suitable way. According to one design, the temperature of the welding electrodes is measured contact-less, especially with means for measuring of infrared radiation, as for example in an infrared camera.
The invention is further described using the attached drawing, which illustrates in its only figure a device for performing the process according to the invention.